[quote name="Jon Isaacs"] [quote] I DO believe a Refractor is the best scope there is. [/quote]

I find that there is no one best scope.A 10 inch f/9 likely ok tor some objects in some situations but the 90 inch focal length makes for a narrow maximum field of view as well as a scope and mount that is difficult to transport and setup. Now if one is lucky enough to live in a black zone with excellent seeing, the manageability is not so important but for those of us who do transport our scopes to darker or more stable skies, there are
scopes that are more practical.

As refractors grow in size they inherently become longer because color correction scales with focal ratio/aperture so that while a 10 inchNewtonian is a reasonable scope to build, a 10 inch f/5 refractor is not
The best scope, the one I happen to be looking through at the moment.

Jon [/quote

Hi Jon,
I believe I answered in detail in my following post why I feel an apo is the very best scope (for me). You may not agree but then you are not me. :^)
The objects I view do not need a wider FOV than it provides. It is a Fantastic telescope and I would not trade it for any other telescope. Period.

"Hi Jim,What was it about the Obsession 20" that you didn't like, compare to the 10" f/9 refractor? "

One of the problems with these forums is that we often speak from our own personal POV and people then try to apply it to their situation which may not apply.
My 10" F/9 TMB is in a roll off roof observatory which is large enough for additional scopes. It is at my Farm which I get to less often than I would like. The sky is dark compared to where I live but not very dark compared to what the committed Deep Sky observer would want. When I get there and the sky is clear I find myself having to choose between the 10" refractor and, say, a 20" reflector. In spite of plans I Always used the 10" refractor. Later, I tried again with an 18" with the exact same result. I Never chose the reflector.

My most frequently used scope is my AP 155mm f/7 EDFS on an AP 900GTO mount. It's fun to use visually. I can see lots of DSOs with it. But, for more detailed views of DSOs aperture, aperture, aperture.

Even for deep sky observing optical quality matters unless we are viewing wide angle objects at 30x.Proper baffling is important and that is why refractors have an advantage over truss systems or reflective tubes.But large dobs make up for that with lots of aperture.Its amazing what a 10" or larger scope shows.Even in the city Planetary Nebulas look good and OIII and UHC filters can work magic.It would take a world class site to get the full resolution out of a 20" mirror.

Even for deep sky observing optical quality matters unless we are viewing wide angle objects at 30x.
Proper baffling is important and that is why refractors have an advantage over truss systems or reflective tubes.
But large dobs make up for that with lots of aperture.
Its amazing what a 10" or larger scope shows.
Even in the city Planetary Nebulas look good and OIII and UHC filters can work magic.
It would take a world class site to get the full resolution out of a 20" mirror.

I use my (80 mm to 180 mm) and A-P 10" Mak-Cass at home (mag 5.8 skies at best), but the 20" Obsession was really quite impressive under mag 6.4 skies in WV when they were transparent (most often the seeing was good but not world-class). I never worried about wringing out the full resolution of that scope. The scope's light grasp and excellent optics provided bright views with tight stars and colorful nebulae. It was a pleasure to use. I need to find a place to use the 25" scope that replaced the Obsession 20".

Sounds fantastic JJK. And, of course, when you head out to the Dark Sky site you only take your big dob. That helps. You don't have to decide which scope to use or what type objects to view. I'd love a peak through that 25"! I'm sure it is fantastic!!

Mirrors. Every time light reflects it doubles the error of the wavefront.

We all know that a 1/8 PV surface error is 1/4 wavefront error effectively doubling all abberations and this is just the primary!

Now the 1/4th wave hits the secondary and doubles the error again to 1/2 wavefront error. If the scope is a straight thru Cassigrain then that's what you get 12",14", 16" highly abberated wavefront. If there is a diagonal involved double the error again.

Sounds fantastic JJK. And, of course, when you head out to the Dark Sky site you only take your big dob. That helps. You don't have to decide which scope to use or what type objects to view. I'd love a peak through that 25"! I'm sure it is fantastic!!

JimP

The 25" is very nice. It really needs to stay at a dark site though. Perhaps I can coax one of my friends with places in WV to house it for me (and them).

I looked through John Vogt's 32" Dob-Newt at the 2012 Black Forest Star Party. The views were stunning. The Swan looked like it was fluorescent.

I was outside of Brussels this past August. One of the professors I had dinner with knows of someone who is making a 1.1 m scope. I think it'll be housed in France. Now there's four excellent reasons to visit there (the art, wine, food, culture, and the 1.1 m)!

I have a question for all you refractor folks.. what would be the minimum aperture (for a refractor) to do decent DSO viewing?

I have plenty of fun using my TV-85 on deep space objects under dark skies. Lately however. I find my 6" F/5 Celestron Omni XLT is getting the most useage since its very compact and portable and 6" is no slouch!!

I'm noticing now, with winter here and temperatures dipping down into the 30's and below, (that's about as cold as it gets out here where I'm at in calif,) my SCT's sit tight in their trunks and my refractors become the scopes of choice to use. The portability and cool down factor plays into this mostly. I wonder if more refractors are sold in the winter time than reflectors or SCT?

I do more solar observing in the winter than spring and summer also, again with refractors, and even my smaller scopes like my TV-85 becomes less of a piggy back scope and more of a main scope when the weather is colder.

We all know that a 1/8 PV surface error is 1/4 wavefront error effectively doubling all abberations and this is just the primary!

Now the 1/4th wave hits the secondary and doubles the error again to 1/2 wavefront error.

If the secondary is of a higher accuracy than the wavefront that hits it, then the error of that wavefront won't be doubled, though it will still be affected. If both mirrors are 1/10th wave on the surface, then the final image will be 1/4th wave on the wavefront.

We all know that a 1/8 PV surface error is 1/4 wavefront error effectively doubling all abberations and this is just the primary!

Now the 1/4th wave hits the secondary and doubles the error again to 1/2 wavefront error.

If the secondary is of a higher accuracy than the wavefront that hits it, then the error of that wavefront won't be doubled, though it will still be affected. If both mirrors are 1/10th wave on the surface, then the final image will be 1/4th wave on the wavefront.

That is at least what my logic tells me.

Clear skies!
Thomas, Denmark

I think if both are 1/10th wave then the result would be 1/5th wave in one orientation and 1/7 wave in the other. This is because of the 45 degree angle on the diagonal.

Of course a refractor has at least 4 surfaces to figure. While the error for each surface is multiplied by n-1, usually a bit more than 0.5. Since there are a minimum of 4 surfaces this adds up to something greater than a factor of 2.

If high quality (or average quality) refractor optics were easily made, they would be cheap. The fact that a decent quality 8 inch achromatic objective from D&G costs about $2400 is indicative of the difficulty. An apochromatic objective is considerably more. For comparison, an 8 inch Zambuto mirror costs $990, a 14.5 inch costs about $2600...

Sure, 1/8th on the primary surface is 1/4 P-V. Then, using a 1/8th diagonal might seem to add another 1/4 wave error to the wavefront making the wave 1/2 P-V before it strikes the eyepiece. Even in the simplest forms, the light must still transit 6 to 8 air to glass surfaces at the eyepiece. Then two more refractive surfaces in the eye. So, by the time the wave hits the retina, it appears to have more than 1 wave of aberration. No one sees this in any star test, which includes the eyepiece.

That level of aberration would make for a terrible view. And since this is not the case with almost every scope, there is a hole in this argument. I just cannot find a source to rebut it, yet.

The answer lies, I believe in the image forming optics as opposed to the flat diagonal. The waveform is made at the primary (reflectors have only one wave forming surface), that is where it varies from perfect reference sphere due to surface error. On the diagonal, the errors might be present, but they do not fundamentally reshape the wavefront. And if they do, it's likely a localized correction within the entire wavefront that may or may not be seen.

Accounting for the wavefront error between 4 to 6 optical surfaces of an APO, even at 1/20th wave smoothness, that would still add up to 4 to 6/20th's = 1/5th to 1/4th P-V error (depending on the type, coverage, and slope of the error, of course, but just assuming smooth SA.) Then that error travels through an eyepiece and they eye, too. So, if the math holds, even the best APO will be below 1/4 wave Raleigh limit.

Again, since this is not true, something is wrong with this argument. Jon may have hit on it above.

In any case, there is nothing wrong with a good parabola. In fact, it can produce the ideal spherical wavefront. Where as highly curved surfaces of APOs and my own Mak have a lot of higher order SA to deal with. Refractors deal with color on top of that.

There are no perfect scopes, just really darn good ones like a good quality refractor. However, reflectors probably have fewer inherent flaws in terms of correction.

I believe the precision of the mirror is typically reported as the wave front error rather than the surface error. Maybe Vla can help us here.

In terms of wave front error in a refractor, I believe Roland Christen uses a standard of 1/8-1/10 wave using an interferometer for his lenses.

The important wave errors are measured at the focal plane at the focal plane because the entire objective contributes to every point on the focal plane so the objective (and Newtonian secondary mirror) errors are summed at every point. The eyepiece only magnifies that image on a point by point basis, bringing your eye closer to the image, the eyepiece does not affect the overall wavefront error.

"Aperture rules???" Well, that is IF you have all the time and money in the world plus a DARK SKY, then it can be so, but not for all folks as we know, I live in mag 3.5 skies and i had an 8inch dob but it was to cumbersome for me to drag it down 3 stairs and out the backyard, plus i had to use it MANUALLY in light polluted skies which is VERY inconvienient so i had to sell it, i have 3 scopes and the biggest is my 4" F7.7 refractor and its a GEM! Its has medium size objective lens, space objects are crisp and sharp especially moon, planets and star clusters, and its not very heavy so i can use it on my DS 2000 go-to mount and serves me very well! So more aperture is better but thats for a rabid deep sky fan that has lots of time and money on their hands plus scopes more than 8 or 10 inches dont work very well in light-polluted skies and here in Jersey transporting a massive scope to a dark site is a MAJOR adventure!

So more aperture is better but thats for a rabid deep sky fan that has lots of time and money on their hands plus scopes more than 8 or 10 inches dont work very well in light-polluted skies

I often use scopes 8, 10, 12.5 inches and even larger from my light polluted back yard... They definitely can out perform my 4 inch apo on the DSOs. Aperture is my friend when it comes to fighting light pollution...

Accounting for the wavefront error between 4 to 6 optical surfaces of an APO, even at 1/20th wave smoothness, that would still add up to 4 to 6/20th's = 1/5th to 1/4th P-V error (depending on the type, coverage, and slope of the error, of course, but just assuming smooth SA.) Then that error travels through an eyepiece and they eye, too. So, if the math holds, even the best APO will be below 1/4 wave Raleigh limit.

Again, since this is not true, something is wrong with this argument. Jon may have hit on it above.

In any case, there is nothing wrong with a good parabola. In fact, it can produce the ideal spherical wavefront. Where as highly curved surfaces of APOs and my own Mak have a lot of higher order SA to deal with. Refractors deal with color on top of that.

There are no perfect scopes, just really darn good ones like a good quality refractor. However, reflectors probably have fewer inherent flaws in terms of correction. [/quote]

This doubling of the error only occurs with mirrors not with refractive surfaces. This might help explain why APOs produce such good images.

As far as the diagnonal, it's a mirror and should double the error but as Jensen mentioned if the diagonal were twice as good as the wavefront off the secondary then the error would be cancelled out. 1/4 wave off the secondary could be truely 1/4 wave to the eyepiece.

wide DSOs benifit from a wide field refractor while faint DSOs benifit from aperture.

there is definitely a place for refractor for viewing DSOs

Yep, use all 3 of my refractors for DSO's both wide-field and high power. In fact much of our video series (Galaxy Log) has refractors being used for many observations, particularly with the use of small and medium size scopes.